Thermoelectric modules, that utilize the Peltier effect to generate heat at one end thereof and absorbs heat from the environment on the other end when electric current flows therein, have been used in cooling applications. The thermoelectric module is envisioned to be particularly promising in such applications as temperature control for laser diode, portable refrigerator, thermostat, photo-detector device and semiconductor manufacturing apparatus. Recently, application of the thermoelectric module to refrigerator and air conditioner for home use is studied vigorously, because of such advantages as the capability to operate without fluorocarbon, vibration nor noise.
A thermoelectric module used for cooling application at a temperature near the normal temperature comprises a plurality of pairs of P type and N type thermoelectric elements connected in series. The thermoelectric module designed for cooling application usually employs thermoelectric elements made from A2B3 type crystal (A represents Bi and/or Sb, and B represents Te and/or Se) for the reason of high cooling performance.
For the P type thermoelectric element, solid solution of Bi2Te3 (bismuth telluride) and Sb2Te3 (antimony telluride) is mainly used and, for the N-type thermoelectric element, solid solution of Bi2Te3 and Bi2Se3 (bismuth selenide) with n-type impurity, namely a dopant aimed at controlling the carrier concentration added thereto is proposed. As the dopant for adjusting carrier concentration, it has been proposed to use at least one kind selected from among a group consisting of Ag, Cu and halogen and halide of Ag, Cu or other metal (see, for example, “Thermoelectric Semiconductor” edited by YOSHIO KAN, Maki-Shoten Publishing Co., Jul. 25, 1966, p 346; Japanese Unexamined Patent Publication (Kokai) No. 1-37456; Japanese Unexamined Patent Publication (Kokai) No. 10-51037; and Japanese Unexamined Patent Publication (Kokai) No. 12-36627).
Such a dopant for adjusting carrier concentration substitutes on Te/Se atom that has different valency and forms a solid solution while releasing electron. The A2B3 type crystal (A represents Bi and/or Sb, and B represents Te and/or Se) is believed to turn into an N type semiconductor when such an impurity is added thereto. A thermoelectric element is made of a thermoelectric crystalline material, and has thermoelectric characteristic that is represented by figure of merit Z. The figure of merit Z is defined as Z S2/ρk where S is Seebeck coefficient, ρ is resistivity and k is thermal conductivity, and indicates the performance and efficiency of the thermoelectric crystalline material when it is used as a thermoelectric element. Higher the value of the figure of merit Z of a material, the higher the cooling performance and efficiency of the thermoelectric module that uses the material become.
The compositions and methods described in the documents cited above, however, there has been such a problem that the N type thermoelectric material has a lower figure of merit than that of the P type thermoelectric material. Therefore, when the P-type and N-type thermoelectric crystalline material are used together to make a cooling element, the element has low cooling performance and low efficiency and cannot be readily applied to refrigerator for home use. Thus there is a demand for great improvement in the figure of merit of the material in order to extend the applications of the material to refrigerators for home use and the like.
As a method for manufacturing the thermoelectric crystalline material from A2B3 type crystal, it has been proposed to crush an alloy, that has been made by melting a mixed powder of Bi, Sb, Te, Se, etc. and solidifying it, into alloy powder and sinter the alloy powder under a pressure by hot press or other process (see, for example, Japanese Unexamined Patent Publication (Kokai) No. 8-32588; and Japanese Unexamined Patent Publication (Kokai) No. 1-106478).
According to the method described in Japanese Unexamined Patent Publication (Kokai) No. 8-32588 and Japanese Unexamined Patent Publication (Kokai) No. 1-106478, the melt-refined alloy is first obtained and is classified by the size of the alloy particles, followed by heat treatment to make it even and sintering under a pressure by hot press or the like. It is claimed that this method enables it to decrease the specific resistance of the sintering material by forming the crystal oriented in the sintered material, so that the figure of merit is improved as the thermal conductivity decreases due to the effect of the grain boundary of the polycrystalline structure of the sintered material.
According to Japanese Unexamined Patent Publication (Kokai) No. 8-32588 and Japanese Unexamined Patent Publication (Kokai) No. 1-106478, however, values of the figure of merit of the thermoelectric materials made from these sintered materials are about 2.8×10−3/K at the best, and the material can be used as a cooling element but has low levels of cooling performance and efficiency, thus restricting its applications and making it difficult to use in a refrigerator for home use. Thus there is a demand for great improvement in the figure of merit of the material in order to extend the applications of the material to refrigerator for home use and the like.
As a method to obtain a thermoelectric material having high value of figure of merit, it is proposed to manufacture an ingot of well oriented crystal or a crystalline material that is proximate to single crystal, through directional solidification based on a known method of manufacturing single crystal such as Bridgman method, pulling (CZ) method or zone-melt method. Since a directionally solidified thermoelectric crystalline material consisting mainly of the A2B3 type crystal (A represents Bi and/or Sb, and B represents Te and/or Se) has the axis of easy crystallization on the a axis, the c plane perpendicular to the c axis becomes parallel to the direction of growth by the directional solidification. In addition, the specific resistance is far lower along the c plane than along the a axis, while Seebeck coefficient and thermal conductivity show less anisotropy, namely less dependence on the crystal orientation. As a result, specific resistance of the directionally solidified thermoelectric crystalline material, when electric current flows along the c plane, can be made much lower than that of a sintered material, thus achieving a higher figure of merit than the sintered material (see, for example, “Thermoelectric Semiconductor and its Applications” Kinichi UEMURA, Isao NISHIDA, THE NIKKAN KOGYO SHIMBUN CO., LTD., Dec. 20, 1988, p 149).
However, the directionally solidified thermoelectric crystalline material such as that described in “Thermoelectric Semiconductor” edited by YOSHIO KAN, Maki-Shoten Publishing Co., Jul. 25, 1966, p 346 has properties resembling those of a single crystal, with the crystal orientation and crystal size being substantially uniform. As a result, means for improving the figure of merit are restricted to adjustment of the composition and optimization of the conditions of directional solidification which, even when carried out, can achieve an insignificant improvement in the figure of merit. Thus attempts to improve the figure of merit of the directionally solidified thermoelectric crystalline material have come to the limit of the prior art.
This limitation is partly due to the high thermal conductivity of the directionally solidified thermoelectric crystalline material. That is, since the directionally solidified thermoelectric crystalline material is constituted from well oriented crystal, there are less grain boundaries and the possibility of phonon dispersion that occurs in a sintered material due to grain boundary is eliminated, thus resulting in high thermal conductivity. As a result, although Seebeck coefficient can be improved and specific resistance can be decreased, thermal conductivity becomes higher at the same time, resulting in insignificant improvement in the figure of merit. Thus improvement of the figure of merit of the directionally solidified thermoelectric crystalline material of the prior art has been limited to about 3×10−3/K due to high thermal conductivity.
It has also been proposed to greatly improve the thermoelectric characteristic by sintering a powder of solid solution that contains antimony under pressure, in such a process as hot press to form a sintered material of high density that cannot be obtained by sintering under normal pressure, with theoretical density ratio of 97% (see, for example, Japanese Unexamined Patent Publication (Kokai) No. 1-106478).
Better mechanical properties than those of the melt-refined materials can be obtained by the use of the hot press process, although improvements of the characteristics are hampered by the oxidation of the stock material powder. To counter this problem, a method of manufacturing a thermoelectric material having uniform particle size and high thermoelectric performance has been disclosed where a heat treatment process is applied to remove fine particles that are more likely to be oxidized from a stock material powder, and the powder of solid solution having particle size in a range from 10 to 200 μm is sintered (see, for example, Japanese Unexamined Patent Publication (Kokai) No. 3-016281).
Also such a method is proposed as a powder of solid solution alloy is made by liquid quenching process, subjecting the powder to reduction treatment in hydrogen atmosphere, and firing the powder under pressure so as to decrease the oxygen content to 1500 ppm or less and improve the thermoelectric performance (see, for example, Japanese Unexamined Patent Publication (Kokai) No. 10-074984).
As a method of removing oxygen that deposits on the surface of the material, it has been proposed to temporarily forming a thermoelectric crystalline material that contains at least two elements selected from among a group consisting of Bi, Te, Se and Sb, calcinate the material at a temperature lower than the firing temperature under a reduced pressure, and apply heat treatment to the calcined material in reducing atmosphere that contains hydrogen (see, for example, Japanese Unexamined Patent Publication (Kokai) No. 9-18060).
Furthermore, it has been shown that, when discharge plasma is generated by applying a voltage directly to a powder so as to activate the surface of the powder and the powder is sintered under a pressure while removing the oxide layer and the adsorbed gas, adverse effect of the adsorbed gas can be reduced and variation in the characteristics of the thermoelectric element can be suppressed (see, for example, Japanese Unexamined Patent Publication (Kokai) No. 5-55640).
However, the thermoelectric material described in Japanese Unexamined Patent Publication (Kokai) No. 3-016281 has such a problem that heat treatment for removing fine particles is applied in the state of powder that has particle size in a range from 10 to 200 μm, and therefore the strength decreases. There has also been such a problem that tendency of the powder to coagulate increases after the heat treatment which results in a longer time taken to insert the powder for sintering and the performance varies depending on the degree of coagulation, while the sintered material thus obtained has lower strength.
The method of manufacturing the thermoelectric material described in Japanese Unexamined Patent Publication (Kokai) No. 10-074984 has such a problem that, in addition to the necessity to sinter the powder, that has been subjected to heat treatment, under a pressure similarly to the case of patent document 2, it is not suited to mass production because the liquid quenching process that requires a special facility is employed, and the characteristic becomes unstable depending on the method of pressurized sintering, thus difference is likely to occur between the inside and the outside.
In the case of the method of manufacturing the thermoelectric material described in Japanese Unexamined Patent Publication (Kokai) No. 9-18060, since the stock material is prepared in a short period of time, quantity of oxygen in the material can be reduced, although a number of processes are involved and the processes are complicated, while performance cannot be improved sufficiently.
In the case of the method of manufacturing the thermoelectric material described in Japanese Unexamined Patent Publication (Kokai) No. 5-55640, since the powder is sintered while removing oxygen that has deposited on the surface of the particles of the stock material powder, though there is an effect of reducing the cost, it is difficult to achieve sufficient reduction of oxygen in the green compact and it is difficult to improve the performance, and the characteristic is unstable with difference likely to occur between the inside and the outside.
Thus with the methods of manufacturing the thermoelectric element of the prior art, the methods are simple but satisfactory performance cannot be achieved, or it has been difficult to obtain a thermoelectric material that has favorable property for mass production and high performance at the same time.
As described above, although the thermoelectric material is required to have the figure of merit greatly improved, thermoelectric materials of the prior art have low values of figure of merit and there have been limitation on expanding the applications of the thermoelectric element made by using the thermoelectric material and the thermoelectric module.
The thermoelectric materials of the prior art also have such a problem that tendency of the powder to coagulate increases after the heat treatment which results in a longer time taken to charge the powder for sintering and the performance varies depending on the degree of coagulation, while the sintered material thus obtained has a low strength.